Heretofore, a fuel cell system including a fuel cell which generates a power by the supply of a reactant gas (a fuel gas and an oxidizing gas) has been put to practical use. It is known that in the fuel cell of such a fuel cell system or a circulation passage of a fuel off-gas, impurities of a nitrogen gas and the like are accumulated with an elapse of time during the power generation. At present, for a purpose of stabilizing the operation state of the fuel cell, there has been suggested the fuel cell system in which a purge valve is provided in a discharge passage connected to the circulation passage, and the opening and closing of the purge valve are controlled to discharge (purge) the impurities to the outside.
Moreover, in recent years, there has been suggested a technology for regulating the open degree of the purge valve so as to keep a nitrogen concentration substantially constant in a fuel gas system (a fuel gas supply passage or the circulation passage) (e.g., see JP-A-2004-185974). The employment of such a technology enables inhibiting the discharge amount of the fuel gas discharged together with the nitrogen gas.